Advanced Sudoku Strategy
Skyscraper Sudoku: How to Find and Use the Skyscraper Technique
Learn how two strong links for one candidate create a Skyscraper pattern. Identify the aligned base cells, find the two roof cells, and remove candidates that can see both roofs.
Two Bases, Two Roofs
Two strong links share one aligned side. The two unaligned endpoints become the roofs, and at least one roof must contain the candidate.
Find Two Strong Links
Choose one candidate that appears exactly twice in each of two rows or columns.
Identify the Roofs
One endpoint from each strong link aligns. The other endpoints become the roof cells.
Remove the Candidate
Remove the candidate from cells that can see both roof cells.
What Is a Skyscraper in Sudoku?
A Skyscraper is an advanced Sudoku candidate-elimination technique based on two strong links for the same candidate.
A strong link occurs when a candidate can appear in exactly two cells within one row, column, or box. Because one of those two positions must contain the candidate, the two cells are strongly linked.
In a Skyscraper, you find two rows that each contain exactly two positions for the same candidate. One candidate position from each row lies in the same column.
Those aligned positions form the base of the Skyscraper. The two remaining candidate positions form the roof cells.
At least one of the two roof cells must contain the candidate. Therefore, any other cell that can see both roof cells cannot contain that candidate.
The Skyscraper Pattern
Two Strong Links With One Aligned Side
The candidate appears exactly twice: once in the shared base column and once in a different roof column.
The same candidate also appears exactly twice: once in the shared base column and once in another roof column.
Remove the candidate from any cell that can see both roof cells.
What Is a Strong Link?
Understanding strong links makes Skyscraper much easier to recognise.
Exactly Two Candidate Positions
If candidate 6 can appear in exactly two cells in a row, one of those two cells must eventually contain 6.
This creates a strong logical connection between the two positions.
A Skyscraper combines two of these strong links for the same candidate.
How to Find a Skyscraper
Choose One Candidate
Select one number from 1 to 9 and examine its possible positions across the grid.
Find a Row With Exactly Two Positions
Look for a row in which the candidate appears in exactly two unsolved cells.
Find a Second Strong Link
Find another row where the same candidate also appears exactly twice.
Check for One Aligned Pair
One candidate position from each strong link should lie in the same column.
Identify the Two Roof Cells
The two unaligned positions are the roofs of the Skyscraper.
Find Cells Seeing Both Roofs
Any cell that can see both roofs cannot contain the candidate.
Remove the Candidate
Delete that candidate from every valid elimination cell that sees both roofs.
Row-Based Skyscraper Example
Suppose you are tracking candidate 5.
Both rows contain exactly two positions for candidate 5.
R2C3 and R7C3 are aligned in column 3. These are the base cells.
Any other cell that sees both R2C8 and R7C6 cannot contain candidate 5.
Why the Skyscraper Elimination Works
The logic comes from the shared base column.
The candidate occupies the aligned position in the first strong link. Because the same column cannot contain the candidate twice, the aligned base in the second strong link is false.
Therefore the second roof must contain the candidate.
The first strong link forces its roof cell to contain the candidate.
So in either case, at least one roof is true.
The Guaranteed Result
At least one of the two roof cells must contain the candidate. A cell that sees both roofs therefore cannot contain that same candidate.
Column-Based Skyscraper
A Skyscraper can also be formed using two columns instead of two rows.
The Same Logic Rotated
Find one candidate that appears exactly twice in each of two columns.
If one position from each column lies in the same row, those aligned positions form the base.
The two remaining positions become the roofs.
Remove the candidate from cells that can see both roofs.
What Does "See Both Roofs" Mean?
A candidate can be eliminated only from a cell that shares a Sudoku unit with both roof cells.
The elimination cell may see one roof through a row or column and the other through a box. The exact geometry can vary.
Skyscraper vs. X-Wing
Because both techniques use strong candidate restrictions, it helps to understand X-Wing Sudoku before practising Skyscraper.
Skyscraper vs. Swordfish
For a larger fish-style candidate pattern, read our Swordfish Sudoku guide .
Skyscraper vs. XY-Wing
For a bivalue-cell technique, continue with our XY-Wing Sudoku guide .
When Should You Look for Skyscraper?
When One Candidate Has Few Positions
Rows or columns containing exactly two positions for one candidate are good starting points.
After X-Wing Searches
If two strong links almost form an X-Wing but only one side aligns, check whether they form a Skyscraper.
In Hard or Expert Sudoku
Skyscraper is useful when simpler candidate eliminations no longer produce progress.
Before Long Chains
Skyscraper is a compact strong-link pattern and can often be found before using longer chaining methods.
When Is It Not a Skyscraper?
❌ A Selected Row Has Three Candidate Positions
The standard Skyscraper pattern relies on a strong link, so each selected row or column must have exactly two positions for the candidate.
❌ No Candidate Positions Align
One endpoint from each strong link must lie on the same opposite line.
❌ You Are Mixing Candidates
All four Skyscraper positions must involve the same candidate number.
❌ The Elimination Cell Sees Only One Roof
The candidate can be removed only from a cell that can see both roof cells.
Common Skyscraper Mistakes
❌ Treating It Like an X-Wing
A Skyscraper does not have two perfectly aligned sides. Only the base positions align.
❌ Eliminating From the Base Column
The key elimination comes from cells that see both roofs, not automatically from every other cell in the aligned base column.
❌ Ignoring Strong Links
Each selected row or column needs exactly two positions for the candidate.
❌ Removing the Candidate From a Roof
The two roof cells create the elimination. Keep their candidate notes and remove only from cells seeing both roofs.
How to Practise Skyscraper
1. Scan One Candidate at a Time
Pick one number and map where it can appear across every row.
2. Find Strong Links
Focus on rows or columns where the candidate has exactly two positions.
3. Compare Two Strong Links
Check whether one candidate position from each link aligns in the same opposite line.
4. Find the Roof Intersection
Search for cells that can see both of the non-aligned roof positions.
5. Reverse Rows and Columns
If you cannot find a row-based Skyscraper, repeat the search using columns.
6. Rescan After Eliminations
A Skyscraper elimination may expose a single, pair, Locked Candidate, X-Wing, or another strong-link pattern.
Skyscraper Practice Checklist
- 1. Are you tracking only one candidate?
- 2. Does the candidate appear exactly twice in the first selected row or column?
- 3. Does it appear exactly twice in the second selected row or column?
- 4. Does one position from each strong link align?
- 5. Did you correctly identify the two unaligned roof cells?
- 6. Does the elimination cell see both roofs?
- 7. Are you removing only the same candidate?
- 8. Did you keep the candidate in both roof cells?
- 9. Did you rescan the puzzle afterward?
Related Sudoku Learning Pages
🧠 Advanced Sudoku Techniques
Explore advanced Sudoku strategies including pairs, fish patterns, wings, strong links, and chains.
✖️ X-Wing Sudoku
Learn the perfectly aligned two-row candidate pattern related to Skyscraper.
🐟 Swordfish Sudoku
Study a larger candidate pattern across three rows and three columns.
🔺 XY-Wing Sudoku
Learn how three bivalue cells create a shared-candidate elimination.
🔒 Locked Candidates Sudoku
Learn how box-line restrictions create candidate eliminations.
🕵️ Hidden Pair Sudoku
Find two candidates restricted to the same two cells in one unit.
Frequently Asked Questions
What is a Skyscraper in Sudoku?
A Skyscraper is an advanced candidate pattern formed by two strong links for the same number. One endpoint from each link aligns, while the other two endpoints form the roofs.
What is a strong link in Sudoku?
A strong link exists when a candidate appears in exactly two cells in a unit, meaning one of those two cells must contain that candidate.
What are the roof cells in a Skyscraper?
The roof cells are the two non-aligned endpoints of the two strong links. At least one of them must contain the candidate.
How is Skyscraper different from X-Wing?
In an X-Wing, both sides align across the same two rows and columns. In a Skyscraper, only one side aligns and the two remaining endpoints create the roof cells.
Can a Skyscraper use columns instead of rows?
Yes. The pattern can be rotated. Two columns can form the strong links while one pair aligns in a shared row.
Does Skyscraper solve a cell directly?
Usually not. It removes a candidate from cells that see both roof cells and may reveal another logical move afterward.
Practise the Skyscraper Technique
Open a Hard or Expert Sudoku puzzle, turn on Notes, choose one candidate, and look for two strong links where one pair of positions aligns and the remaining two cells form the roofs.